In the kraft cooking process, cellulosic material, most conveniently in the form of chips, is treated at elevated temperatures, typically from about 160° C. to 180° C., with alkaline cooking liquor containing sodium hydroxide and sodium sulfide. The fresh inorganic liquor is referred to as white liquor, and the spent liquor containing the dissolved wood material is referred to as black liquor.
Since the emergence of such kraft cooking processes, and to the present date, one of the most important objectives has been the attempt to reduce the energy consumption of the cooking process. Processes have therefore been developed for the purpose of, among other aspects, energy saving. In continuous processes, this may take place by heating the chip material with secondary steam which is obtained from flashing the hot black liquor. In batch cooking processes, the most useful technique is to recover the hot black liquor at the end of the cooking stage, and to reuse its energy 1) as a direct heating medium to be pumped into the digester during a subsequent batch, and 2) to heat up white liquor by means of heat exchangers. Good examples of these developments are batch processes described in, e.g. Fagerlund, U.S. Pat. No. 4,578,149 and Östman, U.S. Pat. No. 4,764,251. The displaced liquors of over 10° C. are stored in one or more pressurized accumulators which further contain a continuous heat recovery system (see, e.g. U.S. Pat. No. 5,643,410). As a result, the energy efficiency of batch cooking has increased.
Another important objective has been to improve the properties and quality of the pulp produced by these processes. In the liquor displacement batch method, avoiding digester discharge by means of hard hot blow techniques has made this possible. Gentle digester discharge is typically accomplished by cooling the digester prior to discharge, relieving the overpressure in the digester and then pumping the cooked material from the digester (see, e.g., U.S. Pat. No. 4,814,042). Further development of liquor-displacement kraft batch cooking has also involved the combination of energy efficiency and efficient usage of residual and fresh cooking chemicals to facilitate delignification and high pulp strength (see, e.g., U.S. Pat. No. 5,183,535 and U.S. Pat. No. 5,643,410). This can be accomplished by arranging the displacement at the end of the cook to first recover the “mother” black liquor, which is hot and rich in residual sulfur, into one accumulator, and then to recover the portion of black liquor contaminated by wash filtrate and lower in solids and temperature in another accumulator. The accumulated black liquors are then reused in reverse order to both impregnate and react with the next batch of wood chips prior to finalization of the cook with white liquor. Thus, it has become possible to start a kraft cook with a high charge of sulfur and a low charge of alkali and carry out important sulfur-lignin reactions in the early phase, especially in the hot black liquor treatment, facilitating subsequent delignification with fresh cooking liquor.
The above-mentioned development of the batch cooking technology has thus been characterized by improvements in terms of energy savings and properties of the delignified cellulosic material such as strength and uniformity.
Also important in the cooking process is that the process have a good fit to surrounding processes as e.g. spent liquor evaporation and pulp washing. Through black liquor evaporation, incineration, melting of the smelt into a water solution and causticizing the resulting liquor, white liquor is regenerated from the chemicals contained in the black liquor. This is the basis for recovery of alkaline spent liquors.
Traditionally, both in the case of batch cooking processes and continuously operated cooking processes, the black liquor led to evaporation originates from the main cooking stage at elevated temperature. In the search for improved energy efficiency and improved properties of the delignified cellulosic material, cooking methods have been developed in which the black liquor fed to the evaporation plant is recycled black liquor originating from the early stages of the cooking sequence. Such processes have been disclosed in e.g., U.S. Pat. No. 5,643,410.
It has been observed that the properties of the black liquor originating from the early stages of the cook differ from those of black liquor from a traditional cook. Recycled black liquor originating from the early stages of the cooking sequence may complicate the evaporation of black liquor. A particular problem is fouling of the surfaces of heat exchangers in the evaporation plant, leading to a decrease in heat transfer. Fouling may be so extensive that the heat transfer surfaces must be repeatedly cleaned, which requires special procedures, calls for an evaporation plant shutdown, and may even limit production. The evaporator fouling problems with black liquors that originates from the early stages of cooking are typically related to calcium.
In the early stage of alkaline cooking, calcium-containing material dissolves into the black liquor from the lignocellulosic material. In a traditional cook, the cook proceeds by heating, the temperature increases and no essential liquor exchange occurs. Then, a major part of the dissolved calcium-containing material in the cooking liquor is broken down, calcium carbonate is formed, and as a result a major part of the calcium is resorbed onto the lignocellulosic material in the digester. Typically, following such a cooking process, evaporation of the black liquor can be carried out without problems caused by precipitation of calcium on the heat transfer surfaces, as the black liquor fed to evaporation originates from the cooking stage at elevated temperatures.
The evaporation problems with black liquors originating from the early stages of cooking typically relate to such calcium-containing material being dissolved in the early stages of a cook. The dissolved calcium-containing material has not been degraded, and the amount of calcium bound to the dissolved material in the black liquor is high. In subsequent evaporation processes, the solids content of the black liquor rises and the evaporation temperature typically increases. Thus, the dissolved material is degraded further as evaporation proceeds and the solids content and temperature both rise. The calcium bound to the dissolved material in the black liquor is thereby set free. The liberated calcium reacts with the carbonate in the black liquor, forming calcium carbonate. A significant amount of crystallization occurs on the evaporation plant's heat transfer surfaces, whereby the plant's water evaporation capacity is severely limited. Crystallization may be so extensive that the heat transfer surfaces must be repeatedly cleaned.
It has been shown that a liquor's potential for calcium carbonate precipitation is dependent on the temperature history of the liquor. Fredrick and Grace (Southern Pulp and Paper Manufacturer 42 (1979) 8:16–23) have proposed that the amount of dissolved calcium is increased because in the black liquor the calcium forms a complex with the lignin. This complex is unstable at higher temperatures. At higher temperatures, the complex breaks down, and if calcium ions are released close to a hot surface, the calcium ion reacts with carbonate ions present in the liquor, and the precipitate is formed on the surface.
Frederick and Grace have proposed that calcium precipitation can be decreased or avoided by heating the black liquor between evaporation stages to temperatures of between about 150° C. to 160° C. and times of from about 10 to 20 minutes. However, the above mentioned method has not been extensively in use because it raises investment and operating costs.
Magnusson, Sjölander and Liden have suggested, that the existence of dissolved calcium in kraft black liquors can be explained by the high amounts of dissolved carbon present (Tappi 1998 International Chemical Recovery Conference Proceedings, Tampa, Fla., USA, 1–4 Jun. 1998, Vol. 1, p. 379–383). The introduction of dissolved forms of calcium into the black liquor will lead to an increased degree of supersaturation. Since this form of calcium is thermodynamically unstable with respect to calcium carbonate formation, heating of such liquors will, at some elevated temperatures, cause rapid precipitation. These authors have shown that calcium carbonate precipitation occurs in the temperature range of from about 110° C. to 145° C., and they have also suggested that potential danger of scaling problems in evaporation plants could be avoided by heat treatment that triggers calcium carbonate precipitation during a process stage where it does not lead to harmful scaling.
Others have proposed direct heating of black liquors fed to the evaporator at a temperature of from about 110° C. to 145° C. and times of from about 1 to 20 minutes (patent application FI 980387). However, this method also increases the investment costs.